This invention relates to a four wheel steering control system for a vehicle. A conventional four wheel steer angle control system is disclosed in an article by Tanaka et al. entitled "Mazda Shasoku Kannougata Yonrinsoda," (Mazda vehicle speed sensing type four wheel steering) 4WS (Yonrinsoda) sha: Active Seigyo Gijutsu no Saizensen, Symposium, Society of Automotive Engineers of JAPAN, 1987. In this conventional system, the a ratio of a rear wheel steer angle to a front wheel steer angle is constant provided that the vehicle speed is constant. Therefore, this conventional system can improve the vehicle stability during a steady state turning motion, but the improvements in vehicle response characteristic are insufficient especially during an abrupt steering operation such as in an emergency, and in a dynamic steering operation such as during travel along a slalom course.
There has been proposed a steer angle control system having a vehicle speed sensor, a steering wheel angle sensor, and a controller for controlling an auxiliary rear wheel steer angle .delta..sub.r (s) with respect to a front wheel steer angle .delta..sub.0 (s) in accordance with ##EQU2## where K, T1, and T2 are control constants. This control system can improve the steering response characteristics and stability of the vehicle.
However, this control system controls only the rear wheel steer angle by controlling a zero side slip angle position in accordance with the vehicle speed. Therefore, the steady state yaw rate gain is uniquely determined, and the flexibility in design of vehicle characteristics is limited. This control system cannot provide a desired yaw rate frequency response.